Remove `skymap_type` for VOEvents
python3 branch includes a few minor changes to the VOEvent file format, including the fact that there won't be any need for the
skymap_type any longer. We should remove it from the API endpoints for creating VOEvents. I'm not sure if you want to remove it from the
VOEvent models - that would remove it for past VOEvents, which might be non-ideal. At least put a comment on the model noting that it's a legacy field.